There are many accurate hygrometers available whose operation depends on such factors as (1) the change in length of a hair or membrane as the R.H. varies, (2) the ability of a hygroscopic solution film to change its electrical resistance with changes in moisture content (R.H.) of the gas above it, (3) the ability of a hygroscopic polymeric sensor to change its electrical resistance with changes in R.H., and (4) the ability of a hygroscopic aluminum oxide film to change its electrical resistance with changes in R.H. There are various types of Psychrometers available, too, and these in general depend on determining the difference between "wet-bulb" and "dry-bulb" thermometer readings to indicate the R.H. of the gaseous environment. There are still other types which depend on electrolyzing the water in the gas stream in order to determine its concentration and thus the R.H. Still others depend on cooling the gas to determine its "dew point", a point which may be related to R.H. with suitable tables. However, all these devices are either mechanically complex, require special manipulation in order to get a reading, require external power, or are delicate in nature. Though these limitations are generally tolerable, there is an acute need for what might be termed a "secondary hygrometer," one easily read, rugged, and independent of external power to serve as a warning device in case the complex devices malfunction. For humidity control is vital in an advanced civilization and great damage may occur if it fails.
There exists a simple, visual type of hygrometer based on the fact that certain cobalt compounds change color from blue to red as the humidity rises. This device is widely used of necessity. But it is very difficult to judge R.H. with any precision using this technique because of the subtle color shifts. Further, the change is of an unobtrusive nature and thus unsuitable for serving as a visual alarm.
Accordingly, one of the objects of the present invention is to provide a visual-type hygrometer in which a number of delineated areas change sequentially from blue-black to brilliant white (or vice versa) as the R.H. rises.
Still another object of the invention is to provide a visual humidity alarm in which a relatively large area changes from blue-black to brilliant white (or vice versa) as the humidity varies from some desired range. In such a device suitable warning legends may appear to the viewer after the humidity has deviated from the desired range.
Another object of the invention is to provide a visual means of determining gas flow patterns in ducts, hoods, etc. by allowing suitably humidified gas to impinge on R.H.-sensitive birefringent crystals deposited on a substrate sandwiched between a perforated, spatially separated, polarizer and analyzer.
Another object of the invention is to provide a special type of hygrometer which can be used as a visual-type thermometer. Delineated areas, for example, may change sequentially from blue-black to brilliant white (or vice versa) as the R.H. falls (in response to temperature rise) in a sealed system having a selected absolute moisture content.
As a variant of this, if an R.H.-sensitive compound which is birefringent at high humidities but non-refringent at lower humidities, is deposited on a suitable substrate and this is sealed into a double-pane window of the type used for insulating purposes, and the whole placed between crossed polarizers, a temperature-sensitive system is obtained. When an appropriate absolute moisture content is sealed in, if direct sunlight falls onto the system the temperature rises, the R.H. drops sufficiently, birefringence ceases, and the system no longer transmits appreciable light.
Another object of the invention is to provide a visualtype moisture tester in which delineated areas change sequentially from blue-black to brilliant white (or vice versa) to indicate the R.H. of the system and thus, secondarily, the moisture content of the material being tested, as paper, tobacco or wood.